Recently, transmission-type and reflection-type liquid crystal display devices, as well as projection display devices using light valves, such as DMD display devices, have become very popular and have become capable of high performance. There is a high demand for utilizing the performance capabilities of these display devices in projectors, including obtaining optical systems in these projectors that enable higher performance to be provided.
For example, a space for inserting a color synthesizing prism for synthesizing light beams from multiple light valves becomes necessary in an optical system using multiple light valves, which, in turn, makes a large back focus distance necessary. Moreover, there arises a need to further improve the image resolution performance of a projection lens. In particular, the deterioration of image resolution in a wide-angle lens due to chromatic aberrations when using projection display devices using light valves has become a problem due to rapid advances in miniaturizing the pixel size and in increasing the number of pixels in light valves.
Lenses described in Japanese Laid-Open Patent Applications 2004-117519 and 2005-62226 have been known as zoom lenses that can ensure a large back focus distance as mentioned above, but these zoom lenses have problems that limit the zoom ratio to values as small as about 1.2, thereby restricting the arrangements of projection display devices. Namely, projection spaces in which the projection display devices are used vary from wide to narrow, and therefore a zoom lens that has a zooming function for changing the magnification of the projected image according to the width of the projection space so as to provide more compatible arrangements has been desired. In this regard, at present, a zoom lens with a zoom ratio of about 1.5 is desired.
Accordingly, as described in Japanese Laid-Open Patent Application 2005-257896, a zoom lens for which the zoom ratio is large has been developed. However, while the zoom lens described in Japanese Laid-Open Patent Application 2005-257896 ensures a zoom ratio of about 1.5, it has a problem of providing a somewhat too-short back focus distance, at least when desiring to insert a large color synthesizing prism.
Also, with the use of high resolution light valves, there are problems in sufficiently correcting the field curvature and chromatic aberrations.